(a) Field of the Invention
The present invention relates to an optical head having optimum tilt angles and, more particularly, to an optical head having optimum tilt angles of the objective lens with respect to different types of optical disk such as CD and DVD.
(b) Description of the Related Art
Along with the development of optical disks, a multiple of types of optical disks are manufactured and used in these days. In view of this fact, the request has increased that a single optical head can record/reproduce data on the multiple types of the optical disks.
Recording/reproducing of data on the typical optical disk is generally performed by accessing a data storage layer of the optical disk through a transparent substrate or transparent layer, which is transparent to the wavelength used and mounts thereon the data storage layer. In order to increase the storage capacity of the optical disk for meeting the request from the market, the data storage layer is improving its performance in the recording density. This generally leads to a smaller-thickness data storage layer in the new type of optical disks for which a new specification is created, in order to allow a minute optical spot to be formed on the data storage layer.
A typical example of the smaller-thickness data storage layer recently achieved is shown by the fact that the conventional compact disk (CD) includes a transparent substrate having a thickness of 1.2 mm, whereas the new digital versatile disk (DVD) includes a transparent substrate having a thickness of 0.6 mm. In this context, the CD has a storage capacity of about 640 MB whereas the DVD has an improved storage capacity of about 4.7 GB.
It is difficult for a conventional objective lens to adapt to different types of optical disks, by forming a focused optical spot having a diameter on the order of diffraction limit thereof, due to the optical disks including transparent layers having different thicknesses.
In view of the above, extensive studies have been conducted to develop a new structure of the objective lens, such as an objective lens having thereon a diffraction grating or having a non-spherical surface. These studies have achieved an objective lens which is capable of forming suitable focused spots on both the optical disks having different thicknesses in the transparent substrates.
There is a remaining problem, however, in the objective lenses thus achieved that it is difficult to manufacture a plurality of objective lenses having an accurate design structure due to the complicated structure thereof which reduces the tolerance in the structure during the manufacturing process.
In particular, the tolerance includes one related to the optimum tilt angle of the objective lens with respect to the optical disk, wherein the optimum tilt angle differs between the different types of the optical disk.
FIG. 1 shows such a problem by illustrating jitter characteristics of an objective lens, wherein the jitter in the reproduced signal is plotted against the tilt angle of the objective lens with respect to the optical disk. In FIG. 1, graph (a) shows the jitter characteristic curve for a first optical disk including a transparent layer having a larger thickness, whereas graph (b) shows the jitter characteristic curve for a second optical disk including a transparent layer having a smaller thickness. As understood from FIG. 1, the optimum tilt angle of the objective lens which achieves minimum of the jitter differs between the different types of the optical disks.
In the conventional technique, the tilt angle is set at an initial angle setting which balances both the characteristic curves. This lowers the tolerance of the optical head related to warp of the optical disk compared to the case wherein the optimum tilt angle is set between the optical disk and the objective lens for the specific optical disk, thereby increasing the wave front aberration.
In view of the above, it is an object of the present invention to provide an optical head which is capable of suppressing generation of the wave front aberration with respect to a plurality of types of the optical disks.
The present invention provides an optical head including: a light source for emitting light having a first wavelength; an objective lens for irradiating the light from the light source onto a data storage layer through a transparent layer in a first optical disk; a lens actuator for mounting the objective lens at an initial tilt angle setting; and a control unit including a calculation section for calculating an offset angle between the initial tilt angle setting and an optimum tilt angle of the objective lens with respect to the first optical disk, and a correction section for correcting, based on the offset angle, a wave front aberration of the light before passing the objective lens.
In accordance with the optical head of the present invention, a plurality of types of the optical disks can be accessed by the optical head with the optimum tilt angles of the objective lens with respect to the optical disks, whereby the jitter characteristic of the optical head can be improved.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.